Television shading signal and clamping circuit



Nov. 3, 1953 H. R. SMITH ,1

TELEVISION SHADING SIGNAL AND CLAMPING CIRCUIT Filed Dec. 50, 1949 2 Sheets-Sheet 1 CIRCUIT UTILIZATION Fig.

VERTICAL SHADING SIGNAL SOURCE INVENTOR. HARRY R SMITH &

ATTORN &

PULSE SOURCE l Nov. 3, 1953 H. R. SMITH 2,658,104

TELEVISION SHADING SIGNAL AND CLAMPING CIRCUIT Filed Dec. 50, 1949 2 Sheets-Sheet 2 45 PERIOD haallkk\.k.k.h

54 1 I I 1' I 1' INVENTOR.

HARRY R. SMITH BY d 2 2141mm; 5

Patented Nov. 3, 1953 IELEVISIQQ SH NG SIGNAL QLAMHN CIRCUIT Harry- It. Smith, Glen Ridge, N. .12, assignor to Allen B. Du Mont Laboratories, Inc., Passaic,

a eneration of elaware pnlieefieu'm cr 30, ,Serial .3 293 2. Glaims.

This invention relates to electronic systems and more particularlyto television transmitter shading signal circuits. I

It is sometimes desirable to employ an iconoscope as a television pick-up device in systems using a circuit to refer, or clamp, a predetermined portion of each horizontal line period to a definite voltage level. Since the iconoscope requires shading signals varying at the vertical as well as the horizontal rate, the vertical shadmg signals would normally be clamped out if they were mixed with the video signals in the manner known the prior art.

Consequently one object of my invention to provide an mprovedtelevision signal genorator.

A second obiect is to provide a method of obtaining vertical shading of a television signal clamped: at a horizontal rate. a

A third object is to provide an improved device for mixing video and shading signals.

A fourth object is to provide a system for using an iconoscope pickup tube in a circuit using a line-rate clamp.

Other-objects of my invention will be apparent j after the specification and: drawings in which:

Figure 1 showsone embodiment of the system;= 1. .1111

2; illustrates typical waveforms obtained at various points of the circuitiof Figure. 1 undernc mel op tes send R i nin in. more shown a pulse voltage source H], which. may be; a television synchronizing generator, connected qtl csaid a ar; amnl fier tube l hav ng: a; thodeb as es s or 2 and, a le e loa resi to is.- A 1 99c. .ube has Pla e t5 on ected to. the p a e of tub H.-

Ibe vsource ll of ver ical. shad ng i na connected. to the atho e. l5 9 diode t rou h a resistor 2. 81. 11 to: th fi iiiiilgs eonteotor L8. of sq entiome en 1- which connected across a ilen e. Qt constant otenti l. e ca ode l6 connected by. means. ofncondenser- M to the aliid; of tube 22.. A resistor- 24 in parallel with a diode Z3: is connected between the grid; of

tube 22' and a source ot' constant potential. resistor 25 connected to the plate of tube 22 serves as a common plate load for horizontal shading signal amplifier 26- and tube 2 2 The plate o e 22 upled: 9v he. g id of a cathode w r ube 21; havin a re i tor 28. n the cat ode. le dihsreot Th athode tot ower tube Z1 is coup ed by means o ndense?- Fi ure. there is.

2 2-9 and potentiometer 39 to the cathode oi tube 3|. A source 3-2 of video signals, such as an iconoscope together with its preamplifier, is connected to the grid of tube 3|. The plate ofthis tube 3| is coupled to the grid of an amplifier tube 33 and the plate of this amplifier tube 33 is turn coupled by means of condenser to the grid of an amplifier tube In accordance with known principier four diodes 35, 31-, 3-8 and '39 term a clamp circuit between the grid of the amplifier tube 35 and a source of constant potential;

The source Ill of pulses to operate the clamp circuit is coupled to the junction point of'the cathodes of diodes 36 and- ;3 l by means of the plate circuit of a phase inverter amplifier tube 40 through a suitable condenser 41 and by means of the cathode of tube 40- through a suitable condenser 42 to the junction of the anodes of, diodes 38 and 39. These pulses generated by pulse source to, are fed; to the grid of the, phase inverter amplifier All through a condenser 43 and across a resistor 44.

The output voltage of the amplifier tube 35 is connected to a utilization circuit 50 such a television transmitter.

The pulses indicated at #5 generated by pulse source HI occur at a frequency equal to the stand; ard television horizontal line frequency and; H a negative excursion. The polarity ,of po e k5 is inverted by amplifier tube H so that they p ea t the plate r: ube 1&5 ne at ve pul es shown at .46.

The efiect of diode ll on pulses, 45 is similar to that of a switch. Solong as the voltage 0;

plate I5 is positive with respect to cathode, t6 the diode l4 forms a low impedance path for the signal current, which is equivalent to saying that the switch is closed. when the; plate i9 is negative with respect to the cathode t6 no u n o s th ugh e d ode sw ch. is there o o en, The voltage, plate 5.1.5 determi ed by h vol age divide action o Plato lo r isto 13 nd tu e M; wh le: the volt e of the cathodeis d termined y: the; olt: a e f t e tap 1.8,- on pote ieme es 9 nd the l ge p a ss resi tor 19- t e will): ag a op 18; s nega iv with rsspectt voltage at the plate of tube H. the ch. is; closed.

The; normal voltage on the plates of tubes H and 4 is peri d c ly r duced by the downward wing-1 nulse 5:- Therefo ein orde tor the iode; 1.4- $9.1. onduct tor the .sn iredu ation Qt the pulses 46, the voltage on contactor I8 must be negative with respect to the voltage appearing at the plate I during peaks of pulses 46 at which times plate I5 assumes its least positive value. Conversely, when it is desired to clip the negative peaks of the pulses 46, it is only necessary to raise the voltage of contactor I8 to such a level that it is positive with respect to the voltage of plate I5 when the pulses 46 cause the plate I5 to assume its least positive value. The voltage at contactor I8 may be varied manually by movement on potentiometer I9 or it may be varied by adding to the D. C. component caused by the voltage divider action of potentiometer IS, a voltage, such as by the sawtooth voltage indicated at 48, generated by the vertical showing signal source IT.

The frequency of the voltage variations of source I1 is equal to the frame frequency of the television signal. Since the pulses 46 occur at a rate equal to the line frequency of the television signal while the fundamental frequency of signal voltage 48 is equal to the frame frequency, signal voltage can modulate the amplitude of the pulses 46. This modulation is accomplished by adding the voltage of signal 48 to the D. C. voltage on contactor I8. As the voltage of contactor I8 and hence of cathode I6 becomes more positive with respect to ground, the amplitude of the pulses 41 appearing at cathode I6 becomes correspondingly smaller, since each time the voltage on the plate of diode I4 decreases, in response to the downward excursion of pulses 46, th absolute voltage level at which plate I5 becomes negative with respect to cathode I6 is higher for each pulse 46 than forthe preceding pulse. At the end of the cycle of signal 48, the voltage on contactor I8 returns to its original value.

There would be no change in the operation of the circuit if signal voltage 48 had any other waveshape than the sawtooth form shown. Likewise, there would be no difference if contactor I8 were moved to a higher voltage point on potentiometer I9 and the signal voltage 48 subtracted from the D. C. component instead of being added. If diode I4 were reversed, it would be necessary to reverse the polarity of signal voltages 45 and 48 in which case a wave having the reverse polarity of waveshape 41 would be obtained. Conversely, if the circuit connections of diode I4 were as shown in Figure 1 but the polarity of pulses 46 were reversed, which might be done either by connecting the plate I5 of diode I4 to the cathode of tube I I or by reversing the polarity of the pulses 45, it would be necessary to reverse the polarity of signal voltage 48 to obtain a wave having the same form as waveform 41, i. e. pulses of decreasing amplitude from the start of the frame to the end, but having the opposite polarity.

The pulses 41 are coupled to amplifier 22 by means of condenser 2|, which removes the D. C. component of the pulse wave. It is desirable to restore the D. C. component of pulse wave 4'! and this is done by means of the D. C. reinsertion circuit consisting of diode 23 and resistor 24 in combination with condenser 2|. Since the condenser 2I need not pass frequencies very much lower than the horizontal line frequency, the time constant of condenser 2| and resistor 24 may be made fairly low, thus excluding to a large extent any power line hum added to pulses 41.

The pulses 4'! are amplified in tube 22 and mixed with the horizontal shadin signal 49 from amplifier 26 to form the combined shading signal 50 which is applied to the grid of a cathode follower amplifier 21 which in turn feeds the combined shading signal to mixer tube 3|.

The signal voltage fed to the grid of amplifier 3I by the video source 32 is indicated by reference character 5I in Figure 2. The signal 5I is shown sloping downward from the start of a frame to the end at which time it returns to its original value. Each line of signal BI is shown as having a sinusoidal variation. Although only two shading components are illustrated in signal 5I-a sawtooth component in the vertical and a sinusoidal component in the horizontal-iconoscopes can and commonly do have sinusoidal, sawtooth, and parabolic components at both the horizontal and vertical repetition rates.

These shading components are so named because they cause the illumination of the television raster to vary from one part of the picture to another. For instance, the components of si nal 5| would cause the television raster to be dim at the top and bright at the bottom as well as dimmer on the left-hand side and brighter on the right.

The composite shading signal is added in mixer tube 3| to the video signal 32 in such a polarity that the line frequency sine wave of the shading signal cancels the sinusoidal component of signal 5| while the amplitude modulated pulses of the shading signal add to the pulses of signal 5I to produce a signal voltage which is amplified by tubes 3I and 33 and applied to the grid circuit of tube 35. The grid of tube 35 is periodically clamped to a fixed reference voltage by a wellknown clamping circuit consisting of diodes 36, 31 and 38 and 39. The pulses controlling the action of the clamping circuit have the same repetition rate as the pulses 45 so that, at the occurrence of each pulse, the grid of tube 35 is returned to a fixed voltage level, thus clamping the vertical shading signals into the television signal 53 which is then passed on to a utilization circuit such as a television transmitter.

If the vertical shading signal 48 were mixed with the horizontal shading signal 49 and the resultant Wave mixed directly with the signal 5I in order to remove the tilt, a signal such as indicated at 54 would result. If this signal 54 were clamped in the same way as signal 52, the effect would be to remove the shading signal 48, leavin the uncompensated signal 5 I.

Although the invention has been described in particular terms, the scope is defined in the appended claims.

What is claimed is:

T. In a television system using a clamping circult to clamp the television signal voltage to a predetermined level during a fixed portion of the blanking interval of each horizontal line period and having a television signal voltage generator which generates a signal voltage having shading components occurring at the vertical repetition rate; a compensating signal generator adapted to generate a signal having the same frequency as said vertical repetition rate; a source of pulses, one of said pulses occurring at said fixed portion of each horizontal line period; a pulse amplitude modulator connected to said source of pulses and to said compensating signal generator for modulatin the amplitude of said pulses in accordance with said compensating signals; a mixer circuit.

connected to said modulator and said television signal voltage generator for combining said television signal voltage and said amplitude modulated pulses; and a clamping circuit connected to said mixer cn'cuit for clam-ping said combined signal voltage to a fixed level at said fixed portion of each horizontal line period.

2. An electronic circuit for compensating the vertical shadin components in a television signal composed of video signals and blanking pulses generated in a television signal generator; said circuit comprising a source of pulses which occur at the same time and for the same duration as said blanking pulses; a compensating signal generator for generating a signal havin a frequency equal to said vertical shading components; a pulse amplitude modulation circuit connected to said source of pulses and to said compensating signal generator for modulating the amplitude of said second named pulses in accordance with the waveform of said generated signal; a mixer circuit connected to said modulation circuit and to said television signal generator for mixing said modulated pulses and said blanking pulses; and a clamping circuit connected to said mixer circuit for clamping the television signal to a predetermined level during the blanking portion of said television signal.

HARRY R. SMITH.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,166,712 Bedford July 18, 1939 2,295,330 Blumlein Sept. 8, 1942 2,298,863 Bartelink Oct. 13, 1942 2,396,865 Longhren Mar. 19, 1946 

